Levels and risk assessment for humans and ecosystems of platinum-group elements in the airborne particles and road dust of some European cities

Abstract

Traffic is the main source of platinum-group element (PGE) contamination in populated urban areas. There is increasing concern about the hazardous effects of these new pollutants for people and for other living organisms in these areas. Airborne and road dusts, as well as tree bark and grass samples were collected at locations in the European cities of Göteborg (Sweden), Madrid (Spain), Rome (Italy), Munich (Germany), Sheffield and London (UK). Today, in spite of the large number of parameters that can influence the airborne PGE content, the results obtained so far indicate significantly higher PGE levels at traffic sites compared with the rural or non-polluted zones that have been investigated (background levels). The average Pt content in airborne particles found in downtown Madrid, Göteborg and Rome is in the range 7.3–13.1 pg m⁻³. The ring roads of these cities have values in the range 4.1–17.7 pg m⁻³. In Munich, a lower Pt content was found in airborne particles (4.1 pg m⁻³). The same tendency has been noted for downtown Rh, with contents in the range 2.2–2.8 pg m⁻³, and in the range 0.8–3.0 and 0.3 pg m⁻³ for motorway margins in Munich. The combined results obtained using a wide-range airborne classifier (WRAC) collector and a PM-10 or virtual impactor show that Pt is associated with particles for a wide range of diameters. The smaller the particle size, the lower the Pt concentration. However, in particles <PM-10, some of the highest values correspond to the fraction <0.39 μm. Considering an average Pt content in all particles of approximately 15 pg m⁻³, which is representative for all countries and environmental conditions, the tracheobronchial fraction represents approximately 10% and the alveolar fraction approximately 8% of the total particles suspended in air. However, from the environmental risk point of view, an exposure to PGEs in traffic-related ambient air is at least three orders of magnitude below the levels for which adverse health effects might theoretically occur (of approx. 100 ng m⁻³). Therefore, today inhalation exposure to PGEs from automotive catalysts does not seem to pose a direct health risk to the general population. Even though the data available today indicate no obvious health effects, there are still a number of aspects related to PGEs and catalysts that justify further research. First, continual monitoring of changes in PGE levels in air and road dust is warranted, to make sure that there is no dramatic increase from today's levels. Secondly, more detailed information on the chemical composition of the PGE-containing substances or complexes leaving the catalyst surface and the size distribution of the PGE-containing particles released during driving will facilitate a more in-depth human risk assessment.

Introduction

Catalysts for automotive traction implemented in all new cars registered in the EU since 1993 contain Pt, Pd and Rh (platinum-group elements, PGEs) for conversion of the pollutants CO, C_xH_y and NO_x into the more innocuous gases CO₂, H₂O and N₂. However, a part of these elements leave the catalyst surface during the lifetime of a catalyst and are transferred into the environment (Palacios et al., 2000). For this reason, traffic is the main source of PGE contamination in populated urban cities. There is increasing concern about the toxic effect of these new pollutants on human beings and other living organisms.

There is a paucity of data on airborne PGE concentrations in traffic-polluted areas. A background level of approximately 2 pg m⁻³ of Pt was found in Germany (Rosner and Merget, 2000) and 0.06 pg m⁻³ of Pd in California (Johnson et al., 1976) before the introduction of exhaust gas catalytic converters. Table 1 summarises PGE contents reported in the literature investigated after the introduction of catalysts. Based on these sparse data, it is difficult to undertake a realistic estimation of the current level of Pt, Pd or Rh in air or to know the size of the particles that contain the PGEs. However, higher concentrations are clearly correlated with a higher traffic density and with time since the introduction of exhaust gas catalysts. For instance, two studies were carried out on road dust collected in the ventilation shaft of a highway tunnel in the north of Graz, near Kapfenberg (Styria, Austria), the first a feasibility study, followed by a certification study. They showed that in 4 years, the time difference between the studies, the average concentration of Pt increased from 55±8 to 81±6 ng g⁻¹ (PACEPAC, 1998). For the future, it can be assumed that the airborne level of Pd will increase by the same order of magnitude if Pt catalysts are to be mostly replaced by Pd catalysts.

The determination of airborne PGEs is very complicated. Neutron activation analysis is very sensitive and accurate, albeit there are few reactors available today. Inductively coupled plasma mass spectrometry (ICP-MS) and differential-pulse cathodic stripping voltammetry (DP-CSV), can be applied for this purpose, but both techniques suffer from severe interference and limitations. In the case of ICP-MS, its advantage over DP-CSV is the multi-element capability and the gain in isotopic information, but spectroscopic interference arising from major constituents of the sample complicate PGE determination. Matrix separation and pre-concentration methods for PGEs in such samples prior to determination by ICP-MS are hampered by the risk of contamination during sample pre-treatment and the lack of quantitative recovery (Schuster and Schwarzer, 1996, Vlasankova et al., 1999, Jin and Zhu, 2000). Applying isotope dilution in order to overcome the lack of quantitative recovery, a necessary condition is the existence of two isotopes free from interference. For Pt (Hf causes interference for all isotopes) and Pd (the main isotopes experience interference from Y, Cd, Cu, Zr and Zn), all isotopes suitable for isotope dilution can experience interference if complete matrix separation is not achieved. Rhodium is a monoisotopic element, and therefore cannot be determined by this method. Recently, a method has been developed for the direct determination of PGEs by laser-ablation ICP-MS in road dust. LA-ICP-MS offers lower oxide interference and contamination risk owing to minimal sample preparation (Motelica-Heino et al., 2001).

The Pt content in road dust, soil and plants growing near roads, all affected by car traffic emissions, has been studied in greater detail than in airborne particles. Table 2 shows the most relevant PGE data in road dust from the literature obtained from different European and non-European cities. The results are not completely comparable, due to the different conditions affecting the PGE content in road dust samples obtained in the different countries, such as road category, weather conditions, place of collection and sampling methodology. However, as in the case of airborne matter, due to the implementation of car catalysts, a clear increase in the PGE concentration is evident and an average Pt concentration in the μg kg⁻¹ range can be considered as an average value representative for most conditions and countries. Additional data to support the assumption of PGE enrichment in road dust and other environmental samples (soils, sediments and plants) have been described by Zereini and Alt (2000). The ability of PGEs to accumulate in grass and other plants under controlled cultivation experiments has also been studied by Schäfer et al. (1998) and Verstracte et al. (1998).

Very little information is available on the bioavailability and toxicity of Pt, Pd and Rh after exposure via inhalation. After intratracheal inhalation of a model substance (Al₂O₃/Pt in a carrier of 0.9% NaCl solution), which is similar to the inhalable particles from catalysts, into laboratory rats, the bioavailability of Pt was investigated. After 90 days inhalation, Pt was found in the blood, urine, faeces and in all important organs, and up to 30% of the finely dispersed Pt was bioavailable (Artelt et al., 1999). For humans, it is not known if people heavily exposed to traffic show increased urinary Pt and Pd excretion or sensitisation (Krachler et al., 1998, Begerow and Dunemann, 2000, Schierl, 2000, Caroli et al., 2001), although it is known that workmen employed in Pt refineries and in Pt catalyst plants show elevated urinary Pt levels (Schierl, 2000).

The investigations reported here are part of a multi-partner project (CEPLACA, 1997), the final objective of which is to assess the human and ecosystem risks based on emission of Pd, Pt and Rh from car catalytic converters. Intensive sampling campaigns for PGEs in airborne and road dust were carried out in selected EU environmental areas for 2 years, with the early results already reported (Rauch et al., 2000, Petrucci et al., 2000, Gómez et al., 2001).